In a conventional lip-type seal installed in a stationary housing and creating a sliding tight seal at the surface of a rotary shaft, in order to address environmental issues, it has been attempted to reduce friction through optimization of the lip shape and dimensions and of the lip material; through coating of the lip sliding surface or of the surface of the shaft paired therewith; or through optimization of the surface roughness of the paired shaft.
A problem encountered with a seal in which the lip sliding surface or the surface of the paired shaft is coated, or with one in which surface roughness of the paired shaft is optimized, is that the effect is obtained only initially, and with the passage of time, reduced friction cannot be sustained due to wear.
The sealing means shown in FIG. 10 (hereinafter “Prior Art 1.” See Patent Document 1, for example) is a known shaft seal device affording satisfactory lubricating characteristics. In Prior Art 1, the design incorporates a lip type seal 51 attached to a housing 50, the lip seal 51 having a sealing edge part 53 arranged so as to contact a rotary shaft 52. A contact zone 54 on the rotary shaft surface contacted by the sealing edge part 53 is equipped with an arrow-shaped grooved part 55 furnished with alternating grooves 56 and ridge parts 57, and as the rotary shaft 52 rotates inside the housing 50, the arrow-shaped grooved part 55 creates a pumping effect, which repels foreign matter infiltrating from the outside atmosphere side, as well as pushing back fluid from the sealed fluid side to maintain a sealing function.
Also known is a shaft seal device furnished with a helical groove in place of the arrow-shaped grooved part of Prior Art 1 (for example, see Patent Document 2).
Furthermore, the sealing means shown in FIG. 11 (hereinafter “Prior Art 2.” See Patent Document 3, for example) is a known shaft seal device for achieving both low torque and sealing properties.
Prior Art 2 has a seal lip 60 for sealing in a sealed fluid, and a screw pump mechanism 64 disposed to the outside atmosphere side from the seal lip 60, and comprising a screw 62 formed on the surface of a rotary shaft 61, and a cylindrical part 63. The screw pump mechanism 64 creates a fluid pumping action towards the seal lip 60, substantially depressing the strained force of the seal lip 60, thereby ensuring sealing properties by the seal lip 60, as well as realizing lower torque of the seal lip 60 through the screw pump mechanism 64.
However, because the structure of the shaft seal device of Prior Art 1 shown in FIG. 10 is such that the sealing edge part 53 of the lip-type seal 51 contacts the arrow-shaped grooved part 55 formed of a high-hardness material furnished in the contact zone 54 on the rotary shaft surface, the sealing edge part 53 wears down quickly. Moreover, the grooves 56 of the arrow-shaped grooved part 55 have a “V” shape through which the sealed fluid side and the outside atmosphere side communicate directly in the axial direction, and the distal end part of the sealing edge part 53 does not contact the “V” shaped grooves 56, whereby the sealed fluid side and the outside atmosphere side are in a state of constant communication, thereby making it possible for the sealed fluid to leak out into the outside atmosphere side when the device is at rest. It is known that in a typical oil seal, shaft surface roughness of 2.5 μm or greater may result in leakage when at rest, and in the case of Prior Art 1, when the depth of the grooves 56 is 2.5 μm or greater, severe leaking will occur when at rest.
In the shaft seal device furnished with a helical groove in place of the arrow-shaped grooved part of Prior Art 1, the shape of the helical groove is such that the outside atmosphere side and the sealed fluid side are in direct contact therethrough, and therefore the design has the same problem as Prior Art 1.
Furthermore, in the shaft seal device of Prior Art 2 shown in FIG. 11, because the strained force of the seal lip 60 is depressed in order to lower the torque of the seal lip 60, the problem of depressed sealing properties of the seal lip 60, and of outside air becoming entrained to the sealed fluid side due to the fluid pumping action of the screw pump mechanism 64, were encountered.